Preheat oven to 350°.,Grease inside of 9 1/2-inch souffl dish.,In medium pan, heat butter till melted.,Add flour, salt, dry mustard and cayenne pepper.,Cook over low heat, stirring constantly until bubbly.,Remove from heat and stir in milk.,Heat until boiling, stirring constantly.,Boil and stir for 1 minute. Stir in cheese till melted, remove from heat.,______,Stir into cheese mixture.,Beat egg whites and creme of tarter into stiff peaks.,Stir 1/3 of egg whites into cheese mixture, carefully fold cheese mixture into egg whites.